The objective is to better understand structure-function interactions in the design and capacity of muscles for blood-tissue exchange. In the previous cycle, we developed methods to assess several aspects of muscle capillary-fiber morphometrics. We propose to use these methods together with histochemical characterization of fiber type and capillary heterogeneities to continue to explore muscle structural capacity for O2 flux, and study its limit(s) and plasticity in relation to fiber demand and distribution. Our studies will mainly focus on muscles of rats, with specific comparison in rabbit, flight muscle of bat and specific muscles of birds, to exploit natural differences in fiber type composition or clustering, capillary geometry and functional constraints. Our specific aims are 1) to assess the upper limits of muscle structural capacity for O2 flux from capillary to fiber mitochondria, 2) determine aggregate surface areas of membranes available for diffusion a) radially from capillary into the muscle fibers, and b) along the arterio-venous capillary path, 3) estimate heterogeneities in fiber type vascularization and structure within and between muscles, 4) compare structural potentials for aerobic function in muscles with large differences in capillary-fiber structure and functional requirements (bird vs mammals; flight muscle vs hindlimb), and assess the plasticity of capillary-fiber geometry in response to 5) chronically increased activity and 6) chronic hypoxia. We will test the hypotheses that 1) the size of the capillary-fiber interface determines O2 flux rates in aerobic muscles, 2) pressure gradients required to drive maximal O2 flux rates are inversely related to membrane surface areas available for diffusion from capillary to mitochondrial enzymes, 3) differences in fiber vascularization and structure are greater between than within muscles, 4) fiber structure and vascularization patterns in muscles of birds and mammals are related to functional demands and constraints, 5) as fiber O2 demand increases with increased activity, the ratio between capillary-fiber interface and aggregate mitochondrial membrane surface areas is critical to establish the limit to O2 delivery and utilization, and 6) the size of the capillary- fiber interface relative to fiber mitochondrial volume increases in response to chronic hypoxia, allowing adequate O2 flux rates to be maintained, at lower Po2 in the capillaries. This will provide new insights in the understanding of key aspects of structure-function correlations in muscle capacity for blood-tissue transfer and should help in the understanding and management of human response to hypoxemia.